Engines may be configured with exhaust gas recirculation (EGR) systems to divert at least some exhaust gas from an engine exhaust manifold to an engine intake manifold. By providing a desired engine dilution, such systems reduce engine knock, throttling losses, in-cylinder heat losses, as well as NOx emissions. As a result, fuel economy is improved, especially at higher levels of engine boost. Engines have also been configured with a sole cylinder (or cylinder group) that is dedicated for providing external EGR to other engine cylinders. Therein, all of the exhaust from the dedicated cylinder group is recirculated to the intake manifold. As such, this allows a substantially fixed amount of EGR to be provided to engine cylinders at most operating conditions. By adjusting the fueling of the dedicated EGR cylinder group (e.g., to run rich), the EGR composition can be varied to include species such as Hydrogen which improve the EGR tolerance of the engine and resulting fuel economy benefits.
Additionally, increased motion of the air and/or fuel charge injected into an engine combustion chamber can increase combustion efficiency under some conditions. For example, charge motion can increase the effectiveness of combustion by introducing air velocity and turbulence in directions perpendicular to the flow direction. By introducing additional kinetic energy into the combustion chambers, the ignition front may traverse the volume of the combustion chamber more quickly and more evenly so as to interact with a heightened amount of fuel before thermal energy is translated to piston motion. Further, resulting turbulence may increase homogenization of the air-to-fuel mixture within the combustion chamber as well as increase the burn rate, which is the time required to for the air/fuel mixture to burn completely during the combustion process. In one example, charge motion control devices, such as charge motion control valves, may be coupled upstream of the intake of engine cylinders in order to increase or decrease the charge motion of a corresponding cylinder, thereby increasing or decreasing the cylinder burn rate, respectively.
In one method for utilizing charge motion control devices, shown by Gopp and Michelini in U.S. Pat. No. 6,715,476, a charge motion control valve or an intake manifold runner control (or similar device) selectively controls the incoming air or air/fuel charge. In one example, a charge motion control valve is placed in the runner associated with each cylinder and is operated by a controller via actuators attached to each respective valve. The actuators can be controlled independently or in a coordinated fashion depending on the particular application. Furthermore, an exhaust gas recirculation (EGR) circuit is provided with an EGR valve to selectively supply a portion of exhaust from an exhaust manifold to an intake manifold. In one embodiment, the EGR exhaust is shown being directed into the intake manifold upstream of where the charge motion control valve is located, such that during operation the air or air/fuel charge along with the EGR exhaust is directed through the charge motion control valve. In this way, determination of the desired EGR flow rate includes the position (open, closed) of the charge motion control valves, among other factors.
However, the inventors herein have recognized potential issues with this charge motion control device and approach. First, the charge motion control valve is described as being open or closed based on a command from the controller, where there is no discussion of the degrees of opening of the valve between the open and closed positions. A range of positions of the charge motion control valve may be desirable for achieving a variety of air-fuel ratios, tumble ratios, and burn rates for each cylinder depending on the engine operating conditions. Furthermore, the EGR system includes an EGR circuit for each cylinder, which directs a portion of exhaust from each cylinder back to the intake manifold. It may be desirable to route a variable amount of exhaust from only a single cylinder (or a dedicated group of cylinders) in order to enhance engine performance while providing more options for charge motion control and EGR.
In one example, the issues described above may be at least partially addressed by a method for an engine, comprising: adjusting each of a first charge motion control device coupled to a first cylinder group and a second charge motion control device coupled to a second cylinder group to vary a charge motion level between the first and second cylinder groups while recirculating exhaust from only the second cylinder group to an intake manifold. In this way, a range of desirable burn rates for the first and second cylinder groups can be achieved via a combination of adjustments of the first and second charge motion control devices and the exhaust gas recirculation from the first cylinder group.
In another example, exhaust from the EGR system can be selectively controlled in two modes; a non-dedicated EGR mode where no exhaust is recirculated into the intake manifold from the first cylinder group, and a dedicated EGR mode where exhaust is recirculated from only the first cylinder group to the intake manifold. During the non-dedicated EGR mode, the first and second charge motion control devices can be adjusted to maintain a common charge motion level between the two cylinder groups. During the dedicated EGR mode, the two charge motion control devices can be adjusted to vary the charge motion level between the two cylinder groups. Other configurations are possible, as discussed in more detail below.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.